The table above is my best attempt at reconciling various tech level tables that have appeared in various games, and then implementing my own. The future tech levels are difficult to adjudicate, since each game has made different assumptions about what will happen.

One point of interest is around the WW2 tech level point. d20 Modern places WW2 as the very end of PL4. GURPS places it at the very end of GTL6. Both merge WW2 with technology found in WW1, The Boer Wars, and the Boxer Rebellion. PL5/GTL7 indicates the extreme high end of things found in WW2 (The USA just about hit this at the very end of the war; others soon followed). Traveller made this two separate tech levels.

Another point of interest is the Cold War era. Traveller split this in two at TTL6 and TTL7. D20 Modern has it as mid PL5, and GURPS has it as GTL7. I'm planning on having the early half of teh era merged with WW2 tech, and the latter half be a new tech level (marked by transistors, PCBs, and other relatively advanced electronics).

What I'm wondering is if my tech levels are too finely-granualted. Thoughts?

I actually think that, for sourcebooks detailing specific eras, you might want to go with fractions (TL0 becomes TL0.0, TL0.1, TL0.2 etc) so that you can show that certain things are advancements within their own time period.

An example of this would be something like RADAR. People can dump metalic particles into the air to try to confuse RADAR operators. But as RADAR technology is improved, it would be able to detect attempts to confuse it.

So maybe someone could do TL10.1 or TL10.7 if they want to have radio and jet engines as different things.

Or maybe you could add in some sort of random number to the end of the tech level to represent the amount of research that people put into an engineering project. That might put something like a jet engine at a target like TL10.6 and a scientist from a TL10 society would then have a 60 percent chance of "discovering" or developing the jet engine. If you added in a random element, like that, it would allow things to be invented in a slightly different order to the real-world, while still keeping things in the same general era.

EDIT: How about if you copy from feats and give "inventions" pre-requisits, as well as TL targets? That would make for a system where you could run through time and randomly roll to see what gets discovered. If anything got missed (or delayed) it might alter what gets discovered afterwards.

First off, forget Traveller future tech levels. In terms of availability of different items, there is no real change from TTL10 to TTL15. All the changes within that band are changes of degree rather than steps that mark new inventions. The only difference between TTL15 and TTL10 is higher levels of efficiency rather than new items. In GURPS terms, Classic Traveller is about GTL10, rising to GTL11 in gravitics and falling to GTL9 in biosciences.

Something interesting happens when you compare what's left. d20 Modern has a "Fusion > Gravitics > Energy > Whatever" paradigm. This happens to be the exact sequence followed by TORG (revised & expanded rulebook being consulted here). The only additions that TORG makes to this is to add macroscopic biotech around the "Energy" level; slow intersteller travel begins at the Gravitics level, and becomes fast at the Energy level.

Interstellar travel is one of those things that is actually tech level independent. In terms of adventure potential, there is no actual difference between a starship travelling at warp 6, a lightsail ship cruising on the solar wind to Mars, a net hacker entering the CR cybernet and using an avatar to hack into a secure complex, a spacecraft entering an Ancients warpgate and arriving moments later around Tau Ceti, or a wizard casting a gate and stepping through to Acheron. The question isn't about whether you want interstellar travel or not, but whether you want "planes" or not, and if so, how those planes manifest. Functionally, a starship, even an interstellar one, is nothing more than a literal wagon train to the stars; in campaign terms, they can be literal wagon trains. GURPS also acknowledged this in their revision from g3 to g4, and their revised tech level table removes the gradations from slowly improving star travel.

In other words, whether you call it Fusion>Gravitic>Energy, or Microtech>Robotic>Exotic, they pretty much line up with each other.

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G3 tech levels seem to have a curious parallel with the Kardashev scale, in which energy consumption determines the tech level. I think this is a fundamentally flawed way of looking at technology levels, as it implies a direct link between having a unified civilisation and having higher technology. There are sufficient tropes in fiction (Mystara's Blackmoor, any post-apocalypse setting, Space 1889, Spelljammer, Star Trek: TOS Organians vs. the Federation, etc.) and gaming that run counter to this idea to make it unworkable. In essence, the scope of a culture is independent of the technology of that culture. TORG made a slight nod in this direction, with separate axioms ("levels") for technology, magic (level of magic possible), faith (level of divine magic possible, eventually switching to a monotheist divine magic level halfway through), and social (degree of social organisation possible).